C++ :

#include<iostream>
#include<cstring>
#define ll long long
#include<set>
#include<map>
using namespace std;

int main(){
	string s;
	int m,x;
	cin>>m>>x;
	cin>>s;
	int sum,num;
	sum=0;
	for(int i=s.length() -1;i>=0;i--){
		if(s[i]=='M'){
			num=i;
			break;
		}
	}
	for(int i=0;i<=num;i++){
		if(s[i]=='M'){
			sum+=m;
		}else{
			sum=sum+(s[i]-'0')*x;
		}
	}
	cout<<sum<<endl;
	return 0;
}

Python :

# coding=utf-8
m,x=map(int,input().split())
s=input()[::-1]
# print(s)

for i in range(len(s)):
    if s[i]!='M':
        continue
    else:
        s=s[i:]
        break
flag = 0
for i in range(len(s)):
    if s[i] == 'M':
        flag += m
    else:
        flag += int(s[i]) * x
print(flag)